This invention relates to a belt-loading roller arrangement, in particular for a high-pressure region in belt-filter presses, comprising at least one belt-loading roller which is embraced over part of its circumference by two belts for enclosing a stock to be pressed.
It is known to apply high pressure to the cake of suspension for the dewatering process by means of filter-presses. Most process, however, demand continuous action such that high pressures may be applied to the cake for a longer period. In paper industry, the known nip rollers are frequently used for the dewatering of paper sludge. "Nip roller system" used in this industry refers to a kind of roller arrangement wherein the rollers may press against one another by appropriate pressing means. Though the nip roller system may provide a high pressure, it acts only for a short period. Normally, the belt-filter presses, especially those used for the dewatering of the sludge cake, adopt the "nip roller system" for merely 5% of the overall process. The reason is that most of the suspensions are too soft to take the high pressure exerted by the nips of respective rollers. The suspension tend to pass through the meshes of the filter belt and squeeze out from the sides of the same.
In an article presented to the Water Pollution Control Federation at the 54th annual conference, Mr. Peterson listed a number of physical and mechanical parameters for belt-filter press. He found that the pressure mainly depends on: (i) the elasticity of the belts, ii) the roller diameter, (iii) the distance between the belts, (iv) the tensioning of the belts, and (v) the thickness of the cake. He thus derived the equation: EQU P=2T/D
where P=pressure to the cake; T=tensioning of the belt and D=roller diameter.
From the equation, it can be seen that pressure increase can be obtained by some combination of increased belt tension (T) and/or decreased roller diameter (D). It should be noted that belt tension arises from three sources, that is:
(a) tension due to drive torque: F.sub.1
(b) tension due to take-up: F.sub.2
(c) tension due to belt elasticity: F.sub.3, and the tension due to belt elasticity is much greater than (a) or (b) above.
Drive torque produces the belt tension required to pull the belt through the press. It is maximum immediately before the drive rolls and may be assumed to decrease linearly through the machine to zero immediately after.
Take-up tension is required to prevent slack belts and to provide traction for the drive rolls. It may be assumed to be constant throughout the machine.
The outer belt circulating the pressure rolls in the press is stretched because it has a greater distance to travel from roll to roll due to the sludge thickness. Stretching the belt requires force, which applies pressure to the sludge cake. The force may be calculated by the relationship: EQU E=F.sub.3 /.epsilon.
where E is the modulus of elasticity of the belt, and .epsilon. is the belt strain in inches per inch.
Regarding tension due to take-up, many designs of belt press use pneumatic or hydraulic cylinders to tension the belts. However, such method is an expensive way to produce cake pressure. It is present throughout the machine and must therefore be resisted throughout. A high F.sub.2 thus requires a costly machine designed. F.sub.3, on the other hand, is present only at the rolls with a cake sandwich, where it is needed, and only these rolls need be designed to resist it. F.sub.3 is not present at drive, take-up, or return rolls.